1. Field of the Invention:
This invention relates to optical systems of changeable magnification, and more particularly to such optical systems in which the image magnification is changed between different values by switching one of the constituent optical units between parallel light paths in cooperation with a path selector.
2. Description of the Related Art:
A wide variety of changeable magnification optical systems capable of selectively employing two lens arrangements for first and second image magnifications have been proposed. Particularly the selection method that is to releasably attach an auxiliary optical unit to either the front or the rear of a master optical unit to change the focal length of the entire system, or the so-called attachment type, because of its operating mechanism being simple in structure, has found many uses in photographic cameras and SV cameras. When the auxiliary optical unit was used as the front attachment, however, its diameter had to be very large. So, the size of the entire lens system tended to increase. For the rear attachment, though its diameter was smaller as compared with the front attachment, the number of lens elements had to be increased, or otherwise good correction of aberrations would have been difficult to perform. Thus, the physical length tended to increase. A common disadvantage of the front and rear attachments was that whenever necessity arose to change the focal length, the auxiliary optical unit had to be either detached or attached or interchanged with another. Because the swiftness to a shutter chance was very poor, snapshots were difficult to take.
In the field of cameras using 110 size film, it has been known to provide a changeable magnification optical system in which the auxiliary optical unit is arranged to slidingly move into and retract from axial alignment with the master optical unit at the front thereof when the image magnification is changed. There has been another proposal in which two lens systems, or the standard and telephoto lenses, are positioned stationary in parallel, and selectively operated by a movable mirror, in Japanese Patent Publication No. SHO 58-38778. The use of such a switch type in the design of changeable magnification optical systems enables a desired decrease in the bulk and size of the entire system to be relatively easily achieved. To focus the first changeable magnification optical system of the switch type, however, regardless of whether the master lens is moved alone, or along with the auxiliary lens, the total focusing movement is caused to differ with different focal length settings. This drawback was also encountered with the second changeable magnification optical system proposed in Japanese Patent Publication No. SHO 58-38778, as the standard and telephoto lenses had different total focusing movements from each other.
This implies that in application of the switch type to auto-focus cameras, for example the operating mechanism must be constructed so as to differentiate the speed of focusing movement of the lens system with different focal length settings, and, therefore that the complexity of the structure of the internal mechanisms of the camera is increased objectionably.